The esophageal mucosa has a number of intrinsic defense mechanisms that retard H+ from penetrating the superficial cell layers and reaching the proliferating cells in the basal layer. When this barrier is overwhelmed H+ diffuses into the deeper cell layer. It is conjectured that the viable replicating cells are protected from H+ by transporters that maintain pH/i. In addition, the mucosa adapts to mucosal acidification by enhancing the rate of epithelial proliferation thereby maintaining the integrity of the mucosa. Recently, several pH/i membrane transporters have been identified in basal cells from rabbits, including an amiloride- sensitive NaH antiporter. The in vivo function of the NaH antiporter in protecting cells from acidification during bouts of acid mucosal reflux is unknown; and it is unclear whether human esophageal cells possess a NaH antiporter. Four molecular isoforms of NaH antiporter have been sequenced, their regional distribution in gastrointestinal tissue of rats and rabbits has been defined and their function has been partially determined. However, none of these studies has identified the type of molecular isoform present in esophageal cells. Studies from our laboratory suggest that esophageal cells from the rabbit, rat and man possess only the NHE-1/NaH isoform. Interestingly, it is the NHE-1 isoform that is important in cell pH regulation and its upregulation has been implicated to play an important role in cellular proliferation. The present study is designed to determine in both man and the rabbit the type of molecular isoform and protein present in esophageal cells. The importance of the NHE-1/NaH antiport in in vivo pH/i regulation will be examined using a chronic rabbit model simulating acid reflux disease and by measuring mRNA abundance NHE-1/NaH in patients with chronic acid-reflux disease. Finally, the cellular and molecular mechanism whereby H+ exposure enhances esophageal proliferation will be examined in an acute model of H+ exposure that has been shown to rapidly enhance cell proliferation independent of mucosal injury. In the kidney, exposure to H+ both in vivo and in vitro has been shown to increase NaH antiport activity through a transcriptional control of the NHE-1 gene and to increase the expression of immediate early genes in a temporal manner similar to what is observed after cells are exposed to mitogenic stimuli. We hypothesize that the acid-induced enhancement of esophageal cell proliferation results from an enhancement in NaH antiport activity and an induction of IE genes. These studies will help clarify how esophageal cells defend themselves against bouts of acidification and how H+ enhances epithelial proliferation at the cellular and molecular level. Also, these studies will begin to unravel the molecular mechanism whereby acid-induced injury causes squamous cells to dedifferentiate into columnar cells.